Composites of porous films useful in a variety of applications such as construction fabric in the form of house wrap are known. Porous melt-processed polymeric films can be made by a number of processes. Among these are processes for stretching films of neat, unblended polymers containing no fillers other than typical stabilizer additives; processes for making films from blends of two or more polymers, or from blends of polymer with mineral oil or an organic salt in which the dispersed phase can be extracted with the film stretched before or after the extraction step; and processes for casting films from polymer blended with a filler such as calcium carbonate or barium sulfate with the film stretched after casting.
U.S. Pat. No. 4,766,029 discloses house wrap material consisting of a three-layer, semi-permeable nonwoven laminate having two exterior layers of spunbond polypropylene fabric and an interior two-component meltblown layer of polyethylene and polypropylene in which the laminate is calendered after formation so that the polyethylene melts and flows to close up the interstitial space and bond the layers together.
U.S. Pat. No. 4,684,568 discloses a process for preparing fabrics suitable for use as roofing-tile underlayment and building air-infiltration barriers that are permeable to moisture vapor and impermeable to liquid water consisting essentially of the steps of applying a continuous coating of polypropylene to a surface of a vapor and liquid permeable, fibrous sheet and then calendering the coated surface.
European Patent Application No. 0 306 818 discloses a process for producing a barrier fabric which is permeable to vapor and impermeable to liquid by laminating a vapor impermeable film to a reinforcing porous fibrous layer such as a spunbond polypropylene fabric to form a composite web and then needling the film of the composite web with tapered needles to form a plurality of micropores through the film.
European Patent Application No. 0 288 257 discloses a flexible membrane useful as lining for roofs or walls made of two spunbond polypropylene layers adhesively bonded together by a discontinuous, porous layer of polyolefin material which is impermeable to liquid water but permeable to air and water vapor.
U.S. patent application Ser. No. 601,519, entitled "Self-bonded Nonwoven Web and Porous Film Composites" filed Oct. 23, 1990, in the name of W. H. Stover, commonly assigned to the same assignee as this application, discloses composites of uniform basis weight self-bonded nonwoven webs and polymeric porous films.
Major limitations of prior art porous film composites comprising layers of microporous film and a second material include the ability to form a porous film composite which retains the vapor permeable and liquid impermeable properties of the porous film while at the same time not contributing greatly to the stiffness or bulk of the composite from the addition of the other material layer. For example, porous film composites wherein spunbond webs are used to impart strength, especially in the cross-machine direction, the spunbond web is typically nonuniform in basis weight and coverage such that relatively "thicker" and "thinner" areas are easily recognized by the human eye. Attempts have been made to compensate for these poor fabric aesthetics and limiting physical properties resulting from the nonuniformity of basis weight and coverage by using spunbond webs having a heavier basis weight than would normally be required by the particular application. This, of course, adds to the cost and contributes to greater stiffness, increased bulk and other undesirable features in the composite.
In view of the above limitations, there remains a need for improved porous film composites wherein the vapor permeable and liquid impermeable properties of the porous film layer are retained with enhanced strength properties provided by a second material layer and, in particular, those composites wherein an oriented polymeric microporous film is adhered to at least one layer of a woven or nonwoven fabric. It is an object of this invention to provide improved composite structures. Another object of this invention is to provide a porous film composite comprising at least one layer of an oriented polymeric microporous film adhered to at least one layer a carded web, a polymeric foam product, a woven fabric, a spunbond fabric, a meltblown fabric or a self-bonded nonwoven web.
The objects of this invention are attained with a porous film composite comprising at least one layer of a highly breathable oriented polymeric microporous film adhered to at least one layer comprising a material which provides enhanced strength and support to reinforce the film without substantially reducing the vapor permeable and liquid impermeable properties of the film layer. Examples of such reinforcing material include a polymeric foam product, a woven fabric, and a nonwoven fabric such as a carded web of staple fibers, a spunbond fabric, a meltblown fabric and a self-bonded nonwoven web. The oriented polymeric microporous film comprises a polymeric composition of an ethylene-propylene block copolymer with an ethylene content of about 10 to about 50 wt %, a polypropylene homopolymer or copolymer of propylene having up to 10 wt % of one or more other .alpha.-olefins of 2 to 8 carbon atoms, and at least one additional component which is a low molecular weight polypropylene, a beta-spherulite nucleating agent or calcium carbonate. For polymeric compositions comprising about 5 to about 30 wt % ethylene-propylene block copolymer, the oriented polymeric microporous film is formed by a process that includes the extraction of beta-spherulites and when the ethylene-propylene block copolymer concentration is about 30 to about 95 wt % the oriented polymeric microporous film is prepared by a process that includes stretching of a heated, nonextracted film.
These composites find particular use for vapor-permeable and liquid-impermeable product applications, for example, air-infiltration barriers, house wrap, roofing-tile underlayment, covers for equipment such as automobiles, motor bikes, stationary equipment and the like, and construction fabric for medical apparel, general protection, chemical protection and the like, and liners for clothing such as sporting apparel and the like. Among the advantages obtained from the porous film composites of this invention are improved hydrostatic waterproofness, improved moisture vapor transmission rate (MVTR) and strength per unit of basis weight, and improved barrier protection from water, solvents, hazardous chemicals and blood. Oriented porous films having a MVTR determined according to ASTM E-96, procedure E, of 500 g/m.sup.2 /24 hr or greater contribute to the improved vapor-permeable and liquid-impermeable properties of the composites of this invention.